The present invention relates to a membrane pressure sensor containing silicon carbide. It also concerns the manufacture of the sensing element of the sensor.
With micro-electronic techniques it is possible to produce miniature pressure sensors using collective manufacturing processes. Small-sized sensors can therefore be produced at low cost. They provide the possibility of producing a sensor and its associated electronics on one same carrier.
Micro-machined pressure sensors are known, made up of a silicon membrane a few tenths of a xcexcm in thickness. The difference in pressure between the two surfaces of the membrane may be detected by measuring the recess stresses by means of piezoresistive gauges obtained by ion diffusion or implantation. These piezoelectric gauges have high sensitivity and extensive mechanical stability due to the monocrystalline structure of the silicon used. Between each gauge and the substrate on which they are fabricated, electric insulation is achieved using reverse junctions. This has the disadvantage of limiting the range of operating temperature of these sensors to a maximum of 125xc2x0 C. due to the strong leakage current at the reverse junction, and of causing a high noise level (thermal noise and piezoelectric junction noise) which reduces the dynamic range. A further disadvantage of piezoresistive gauges for their usual use results from the direct exposure of these gauges and from metallisation related to the fluid whose pressure is to be measured, which submits these elements to the effects of humidity and corrosive agents.
Pressure sensors are also known with piezoresistive gauges embedded in the silicon. However, these sensors cannot be used for temperatures over 200xc2x0 C.
Pressure sensors made on SOI (Silicon-On-Insulator) substrates are also known. These sensors do not have the disadvantages due to leakage current or to noise on account of the intermediate insulating layer. They may be used up to temperatures in the order of 400xc2x0 C.
Research is currently focusing on techniques using silicon carbide, which are able to provide products able to operate up to temperatures in the order of 700xc2x0C.
These micro-machined pressure sensors are encapsulated in relation to their intended use. Pressure sensors for the automotive industry and those intended to be mounted on a board are generally encapsulated in pre-moulded xe2x80x9cdual-in-linexe2x80x9d type casings. Some casings may be custom-made in relation to the intended application, by designing a pre-moulded housing in thermoplastic material to ensure the best possible mechanical integration of the sensor, and by using the xe2x80x9cdual chipxe2x80x9d technique for the incorporation of associated electronics. In practice, this type of encapsulation offers very few application possibilities and the presence of thermoplastic material imposes a maximum temperature.
The use of pressure sensors in a hostile environment requires giving consideration to the restraints of temperature, and the type of fluid whose pressure it is sought to measure, in particular its corrosive nature. So as to protect the membrane of the sensor from its immediate environment, encapsulation frequently integrates means for the hydraulic transmission of the pressure to be measured, combining for example silicon oil and a membrane or a metal bellow device. This solution has the disadvantage of increasing the cost of the sensor. Also, silicon oil does not withstand a temperature greater than 300xc2x0 C. For higher temperatures, mercury may be used, but regard must be given to its harmful effect on the environment.
In a hostile medium, encapsulation may further use materials such as stainless steel and ceramic in order to protect the silicon part of the sensor. The sensor membrane may be protected from the medium, whose pressure is to be measured, by another membrane which directly covers the first membrane (enabling operation of the sensor up to 300xc2x0 C.) or by a mechanical pressure transmission system using a diaphragm (which enables operation of the sensor up to 450xc2x0 C).
U.S. Pat. No.4,898,035 discloses a ceramic pressure sensor, for the measurement in particular of the pressure in the cylinder of an internal combustion engine. This sensor comprises a sensing element integrating a membrane, a first surface of the membrane being intended to contact the medium whose pressure is to be measured. The second surface of the membrane supports membrane deformation detection means connected to electric conductors, not shown. The membrane being is ceramic, its surface intended to contact the hostile medium is chemically inert relative to this medium. The carrier of the sensing element supports this element so that one of the surfaces of the membrane is in contact with the hostile medium and the opposite surface is shielded from this contact. The carrier and ring disk are metallic. The seal between the inside of the sensing element and its carrier is ensured by the connection layer in glass or a brazing material.
U.S. Pat. No. 4,894,635 discloses a stress sensor, for example a pressure sensor. This sensor is intended to operate at high temperature. It is also intended for use in a hostile medium such as vehicle engines. The sensing element is formed from a substrate in ceramic. It comprises a membrane of which one surface is exposed to the medium whose pressure is to be measured, and the other surface supports the detection means. Parts act as support for the sensing element. They place one of the membrane surfaces in contact with said medium and prevent the surface opposite the membrane from coming into contact with this medium. The seal is ensured by a toroidal joint.
Document DE-A-196 01 791 discloses a membrane detector and its method of manufacture. The detector is a micro-machined structure comprising a deformable membrane integral with a peripheral part enabling its deformation. The membrane comprises a layer in SiC and a layer in SiO2. The detection elements are placed on the electric insulating layer.
U.S. Pat. No. 4,706,100 discloses a piezoresistive pressure sensor comprising: a substrate of monocrystalline silicon, an epitaxied layer of monocrystalline xe2x96xa1-SiC, piezoelectric resistances formed by diffusion or implantation in the epitaxied layer, electric contacts to connect the piezoelectric resistances and a cavity formed on the rear surface of the substrate to form a membrane.
The present invention was designed to remedy the disadvantages of pressure sensors of the prior art. It can be used to produce a miniature pressure sensor manufactured by collective manufacturing processes, compatible with resistance to a severe environment (high temperature, chemically aggressive measuring medium), compatible with simplified encapsulation and having a low production cost.
The subject of the invention is therefore a pressure sensor able to operate at high temperature and to measure the pressure of a hostile medium, comprising:
a sensing element integrating a membrane in monocrystalline silicon carbide and produced by micro-machining a substrate in polycrystalline silicon carbide, a first surface of the membrane being intended to be placed in contact with said medium, a second surface of the membrane comprising means to detect membrane deformation connected to electric contacts for connection of the electric connection means, the surfaces of the sensing element intended to be in contact with said medium being chemically inert relative to this medium;
a carrier supporting the sensing element so that said first surface of the membrane may be contacted with said medium and the second surface of the membrane may be shielded from contact with said medium, the carrier being in polycrystalline silicon carbide;
a seal strip in material containing silicon carbide brazed between the carrier and the sensing element to protect the second surface of the membrane against any contact with said medium.
If the sensor is intended to measure absolute pressure, the carrier may comprise a sealed closing part so that a vacuum can be set up inside the carrier.
Advantageously, the carrier is tube-shaped, the sensing element closing one of the tube ends, the first surface of the membrane being directed towards the outside of the tube. It may then be provided with a thread with which it can be screwed onto a reservoir containing the medium.
An insulating interface layer may be provided between the membrane and the substrate part of the sensing element. This insulating interface layer may be in a material chosen from among silicon oxide, silicon nitride and carbon-containing silicon.
The electric contacts equipping the detection means may be in a silicide containing tungsten. The connection between the electric contacts and the electric connection means may be obtained by a solder material withstanding high temperatures. This solder material may be a silicide containing tungsten. Conductor means may also be provided forming a spring to ensure the connection between the electric contacts and the electric connection means.
The detection means may comprise at least two piezoresistive gauges, in monocrystalline silicon carbide for example.
A further subject of the invention is a method of manufacture by micro-machining at least one membrane sensing element for a pressure sensor able to operate at high temperature and to measure the pressure of a hostile medium, comprising the following steps:
a) producing a layer of monocrystalline silicon carbide on one surface of a substrate containing polycrystalline silicon carbide,
b) fabricating, on the free surface of the monocrystalline silicone carbide layer, means to detect membrane deformation,
c) fabricating electric contacts on said free surface to connect the detection means to the electric connection means,
d) forming the membrane of said sensing element by removing material from the other surface of the substrate, so as only to preserve polycrystalline silicon carbide.
The fabrication of said layer of monocrystalline silicon carbide may comprise:
transferring a first layer of monocrystalline silicon carbide onto said surface of the substrate,
depositing by epitaxy a second layer of monocrystalline silicone carbide on the first layer in order to obtain said monocrystalline silicon carbide layer of controlled thickness.
The production of said layer of monocrystalline silicon carbide may entail the use of a wafer in monocrystalline silicon carbide in which a layer has been defined by a layer of microcavities generated by
depositing by epitaxy a second layer of monocrystalline silicone carbide on the first layer in order to obtain said monocrystalline silicon carbide layer of controlled thickness.
The production of said layer of monocrystalline silicon carbide may entail the use of a wafer in monocrystalline silicon carbide in which a layer has been defined by a layer of microcavities generated by ion implantation, said wafer being bonded to said surface of the substrate and then cleaved at the layer of microcavities so as only to preserve said layer defined on the substrate. Preferably, cleavage of the wafer is obtained by coalescence of the microcavities resulting from heat treatment. Also preferably, the bonding of said wafer onto the substrate is obtained by molecular bonding.
Before the step to produce said layer of monocrystalline silicon carbide, an insulating interface layer may be deposited on the surface of the substrate on which said layer is to be made.
During the membrane formation step, the removal of matter from the other surface of the substrate may be conducted using an operation chosen from among mechanical machining and chemical etching.
According to one variant of embodiment, the method may comprise the following preliminary steps:
machining a substrate to obtain a bump of complementary shape to the shape of the desired sensing element as seen from the hostile medium side,
depositing a layer of polycrystalline silicon carbide on the substrate on the bumping side,
levelling the previously deposited layer down as far as the tip of the bump, steps a) and d) then being conducted in the following manner:
a) the layer of monocrystalline silicon carbide is formed on the substrate on the levelled layer side,
d) the membrane of said sensing element is formed by removal of the initial substrate.
The layer of chemically inert material deposited on the substrate on the bumping side must be sufficiently thick to ensure good mechanical resistance when the initial substrate is removed.
The substrate may be in silicon.
The fabrication of said layer of monocrystalline silicon carbide may comprise:
transferring a first layer of monocrystalline silicon carbide onto the substrate,
depositing by epitaxy a second layer of monocrystalline silicon carbide on the first layer of monocrystalline silicon carbide in order to obtain said monocrystalline silicon carbide layer of controlled thickness.
Advantageously, the depositing step of a layer of polycrystalline silicon carbide may be made by CVD for example. The levelling step may be conducted by mechanical-chemical polishing.
According to this variant of the method, the production of said layer of monocrystalline silicon carbide comprises the use of a wafer in monocrystalline silicon carbide in which a layer has been defined by a layer of microcavities generated by ion implantation, said wafer being bonded to the substrate on the side of the levelled layer then cleaved at the layer of microcavities so as only to preserve said layer defined on the substrate. Preferably, cleavage of the wafer is obtained by coalescence of the microcavities resulting from a heat treatment. Also preferably, the bonding of said wafer to the substrate is obtained by molecular bonding.
Before the step to produce said layer of monocrystalline silicon carbide, an insulating interface layer may be deposited on the surface of the substrate on which said first layer is to be formed.
During the membrane formation step, the removal of the initial substrate may be obtained by chemical etching.
According to another variant of embodiment, the method may comprise the following preliminary steps:
machining a substrate to obtain a bump of complementary shape to the shape of the desired sensing element as seen from the hostile medium side,
depositing a layer of polycrystalline silicon carbide on the substrate on the bumping side,
levelling the previously deposited layer so that above the bumping only the desired membrane thickness subsists, steps a) and d) then being conducted in the following manner:
a) the layer of monocrystalline silicon carbide is formed on said levelled layer,
d) the membrane of said sensing element is formed by removal of the initial substrate.
The substrate may be in silicon.
Advantageously, the depositing step of a layer of polycrystalline silicon carbide may be made by CVD for example. The levelling step may be conducted by mechanical-chemical polishing.
According to this other variant of embodiment, the fabrication of the layer of monocrystalline silicon carbide may be obtained using a wafer in monocrystalline silicon carbide in which said layer has been defined by a layer of microcavities generated by ion implantation, said wafer being bonded to the substrate on the side of the levelled layer then cleaved at the layer of microcavities so as only to preserve the layer of monocrystalline silicon carbide on the substrate. Preferably, cleavage of the wafer is obtained by coalescence of the microcavities resulting from a heat treatment. Also preferably, the bonding of said wafer to the substrate is obtained by molecular bonding.
During the membrane formation step, the removal of the initial substrate of silicon may be obtained by chemical etching.
An insulating interface layer may be deposited on the levelled layer before placing said layer of monocrystalline silicon carbide. During the forming of the detection means, the remaining part of the monocrystalline silicon carbide layer may be removed.
If the method of the invention is a collective method for fabricating sensing elements from one same substrate, a final substrate cutting step may be provided to obtain separate sensing elements.